Need some closure. 66cc top end failure.

[WRENCHBENDER]

Well I have about 93 miles on my engine and I ride it alot and one day on my way home from work the motor suddenly started to miss real bad and then it just shut down. while i was coasting with the clutch out i could hear the oh so familiar sound of engine failure. pedaled the rest of the way home and immediately started to tear down the engine. First bad sign was aluminum peices rattaling in the exhaust pipe. so what happened was the wrist pin had traveled past the side or the piston and tore a big hole in the transferr port. Peices of metal and the c-clip were all over and smashed the spark plug shut. but was it my fault. I have modded the engine a little. port matched the intake and exhaust,re-jetted the carb from .030 to .025 and it ran alot better and could get some more rpm from the engine, no more four strokin. Dont get me wrong I wasnt babying it. I started to get real bad vibrations when i pushed it over 32 mph. I got up to 34.9 mph on a 44 tooth gear if my speedo was correct and i know i have gone over 30 mph. It would vibrate so bad over 34 mph that the gas would leak from the tank neck. However the material between the wrist pin clip and the side of the piston was very thin in my opinion and was thicker on the other side. Was it a failure of the clip or did the metal of the piston give way and lost the clip. Has anyone put internal c-clips on the piston or whats a good replacement tip. so let me know your input and now im in the market for a new top end. Also i was still running the 16:1 ratio and have changed the spark plug to a better n.g.k. plug that was running fine for a long time.

 

[GearNut]

When moderately modded, The wrist pin clips are known to fly out, the small end bearings are known to disintegrate, and the wrist pin bosses in the pistons are known to shatter.
These engines really cannot take serious horsepower modding. They are just not built out of good enough material to handle it. They are Chinese, not Japanese engines; Ain't no Hyabusa!
I am sorry to hear that yours went down in a typical fashion. That's alot of work just to have blow up underneath you.
210061741 has taken them to the limit and now plans on going whole hog with using moped parts for reliability, when he can afford it. Parts are not cheap when they keep blowing up.
http://www.motoredbikes.com/showthread.php?t=27248

 

[scotto-]

That says it all...

GearNut nailed it to a "T", couldn't have said it better if I tried. Parts are cheap, build er back up and break it in easy, cheers!

 

[AussieSteve]

This engine, by the sound of it, was only port-matched and a smaller main jet, just like mine. That shouldn't cause the gudgeon pin to fly out.

Possibly the gudgeon pin 'C' clip had been squeezed in a bit too much during assembly and wasn't a firm fit in the groove. That's the most likely cause.

 

[GearNut]

This wasn't a 'seriously modded' engine, by the sound of it - only port-matched and a smaller main jet, just like mine. That shouldn't cause the gudgeon pin to fly out.

Possibly the gudgeon pin 'C' clip had been squeezed in a bit too much during assembly and wasn't a firm fit in the groove. That's the most likely cause.

I was not trying to say that he tried to make a 10 horsepower engine out of it.
I will edit my post to more accurately explain my thoughts.
I agree with your evaluation of the cause of failure.

 

[WRENCHBENDER]

Search for better/reliable parts.

I plan on rebuilding it with some better components but, whats a good combination to use in the rotating assembly. I have diassembled the entire engine to clear out all the debris from the lower end and want to install better, more reliable parts. Where and what has been proven reliable. I have been brain storming on having my old vocational school machine shop do a little modifying on the crank but, what about a forged piston.

 

[GearNut]

Moped cylinders and pistons come to mind, but then the connecting rod and flywheel assembly become the next weak link.
Check out 210061741's threads.
Here's one to start with: http://www.motoredbikes.com/showthread.php?t=26980
Unfortunately there is alot of "bashing of heads" that happen in the middle of it.
The information shared there is greatly appreciated (at least by me) and, if you overlook the unpleasantrys, it is a good read

 

[AussieSteve]

Gearnut, that particular thread is a bad example. There's no engine info whatsoever, just one big s..tfight.
Still, amongst Rich's other posts is info on part numbers and sizes for replacement bearings and seals - a must. (Crankshaft bearings and seals, big-end and little-end.) The little-end needle bearing is notorious for failure, with the big-end just behind it.
Not a bad idea to replace the countershaft bearings and seals as well.
Regarding crankshaft mods, the best thing you can do is balance the crank, especially on the 66cc engines. The bottom-end is balanced for a 48cc top-end. That's one of the reasons 66cc engines vibrate more and have trouble revving out.
Possibly search for 'bearings' and you might find the right posts.

 

[GearNut]

In the very beginning, before the poo starts flying, 210061741 shares links to moped cylinder and piston kits and he even has a video sharing his thoughts and ideas.
I also wanted to introduce 210061741's name so WRENCHBENDER could search his threads to learn a awesome information concerning "outside the box" upgrades that can be done.

 

[WRENCHBENDER]

Well it looks like I got some serching to do. I have been contemplating about balancing the crankshaft but should I do it with or without bob weights. I know the flywheels have voids in them for balance. Has anyone thought about some crazy ideas about crankshaft modifying. For instance I was thinking if i took the crankshaft to the machine shop and have cuts made into new flywheel weights to simulate like a "squirrel cage" fan that accelerated the flow through the bottom end up into the transferr passages. A little calculation would have to came into play here to about what degree of pitch the cuts would have to be but, it would work something like a internal turbo. Or even stroking the crank, like moving the conecting rod pin another .025" from current center. This would increase compression but, would require some port work in the cylinder. These ideas would proably blow up in my face but hey, got to keep my mind busy.